Radiotracers of various nonelectrolytes were injected intravenously in conscious rats, and arterial plasma concentrations were determined as a function of time until the animals were killed by decapitation. Regional brain concentrations of tracer were measured. Nonelectrolyte permeability at the blood-brain barrier is due to simple diffusion through the cerebral vasculature, and is comparable to permeable across nonporous, bimolecular lipid membranes.